1. Field of the Invention
The present invention relates to a polymer-based optical waveguide circuit stable to the changes of ambient temperature which is profitably used as a wavelength division multiplexer or demultiplexer in, for example, fiber optic communication.
2. Description of the Related Art
In a dense wavelength division multiplexing (DWDM) system applied to optical carriers belonging to S, C and L bands, the wavelength independent waveguide coupler has been used that is capable of routing optical carriers comprising 80-100 nm bands around 1550 nm into two channels at a specified split ratio, being unaffected with their wavelengths. The coupler that routes optical carriers at a split ratio of 1:1 is called a 3 dB coupler and has been used in multiple fiber optic communication systems (K. Jinguji et al., xe2x80x9cMach-Zehnder interferometer type optical waveguide coupler with wavelength-flattened coupling ratio,xe2x80x9d Electron. Lett., 1990, Vol. 26, No. 17, pp. 1326-1327).
Such a wavelength independent waveguide coupler includes a Mach-Zehnder interferometer type optical circuit 17 as shown in FIG. 8. The circuit comprises two waveguide cores 13, 14 formed on the surface of a substrate 20 prepared from a quartz or silicon wafer.
The waveguide cores 13, 14 are covered with lower and upper cladding layers 18, 19 both of which are formed on the substrate 20. When the core and cladding layers are mainly composed of silicon dioxide (SiO2), the resulting optical circuit is called a quartz waveguide. When they are composed of a polymer, the resulting optical circuit is called a polymeric waveguide.
The Mach-Zehnder interferometer type optical circuit 17 comprises two directional couplers 15, 16 which are obtained by bringing the two waveguide cores 13, 14 close to and in parallel with each other. The Mach-Zehnder interferometer type optical circuit 17 receives an optical signal having a band width of 80-100 nm around 1550 nm either from a terminal 13a or 14a connected respectively to the waveguide core 13 or 14 and splits the signal at a split ratio of 1:1 to deliver two outputs each having an intensity half that of the transmitted signal from terminals 13b, 14b, or the other terminals of the waveguide cores 13, 14.
An application of such a Mach-Zehnder interferometer type optical circuit includes an interleaver as shown in FIG. 2. This interleaver with two polymeric waveguides 9, 10 comprises circuits 11, 12: each of the circuits is equivalent to the Mach-Zehnder interferometer type optical circuit 17 shown in FIG. 8, and the two circuits are arranged to each other in a point symmetrical manner. If it is required to separate wavelengths having a bandwidth of 100 nm, it is only necessary for the interleaver shown in FIG. 2 to be configured such that a proper optical path difference is inserted between the two waveguides 9, 10. This simplifies the designing of the interleaver.
For example, according to the interleaver shown in FIG. 2, it is possible by designing the waveguides 9, 10 so as to produce a proper optical path difference, to receive optical signals xcex1, . . . , xcexn each having a band width of 80-100 nm around 1550 nm from a terminal 9a of a waveguide 9, separate those signals according to their wavelengths, route them alternately into two channels, and deliver them as separate outputs from the other terminals 9b, 10b of the waveguides 9, 10.
However, because the interleaver shown in FIG. 2 includes the waveguides made from a polymer, its optical characteristics are apt to vary in the presence of changes of the ambient temperature, and thus the temperature range under which it can normally operate is rather limited. The temperature coefficient of the refractive index of a polymer material used for the construction of the waveguides is ten or more times as high as that of quartz. Therefore, if the ambient temperature changes, the polymeric waveguides 9, 10 and cladding layers covering those waveguides will undergo a great change in their refractive indices; the parameters of the optical circuit including those waveguides and cladding layers will also shift from the designed ranges, and the performance of the optical circuit will depart from the designed level. Degraded performance of an optical circuit with polymeric waveguides as a result of the alteration of ambient temperature is mainly ascribed to the following two reasons. The first reason is: a phase difference which is produced as a result of properly chosen path length difference between the two waveguides 9, 10 is modified in the presence of a change of ambient temperature, which in turn brings about a change in the central wavelength of the affected optical carrier. The second reason is: an optical carrier having passed the Mach-Zehnder interferometer type optical circuits 17 exposed to a change of ambient temperature is modified such that its wavelength shifts to a shorter or longer one. For example, the refractive index of a polymeric waveguide has a temperature coefficient of aboutxe2x88x92(1.1-1.8)xc3x9710xe2x88x924/K, and if such a polymeric waveguide is exposed to a temperature change of 40xc2x0 C., a carrier having passed through the waveguide will undergo a shift of about 6.5 nm in its wavelength. Assume that an interleaver with polymeric Mach-Zehnder interferometer type optical circuits separates carriers at 0.8 nm intervals and is exposed to a temperature change of 40xc2x0 C., and thus a carrier having passed the Mach-Zehnder interferometer type optical circuits suffers a shift of about 6.5 nm in its wavelength. Then, the carrier will be routed to a channel by eight channels shifted from the one to which it should be routed. According to an experiment, if such an interleaver as above is exposed to a temperature change of 40xc2x0 C., a carrier having passed through the Mach-Zehnder interferometer type optical circuits 11, 12 is modified so much that its output changes by xc2x12% or more at the ends of its band width, and that it is impossible to maintain the crosstalk between adjacent carriers at xe2x88x9230 dB or lower.
The first problem will be solved by adjusting the physical parameters of waveguides 9, 10 such that they satisfy the following equation for a wavelength having a given bandwidth:
∂xcex210/∂T=∂xcex29/∂T(L9/L10)xe2x80x83xe2x80x83(1)
where xcex29 and xcex210 represent the transmission constants of waveguides 9 and 10 for a mode of optical carriers, L9 and L10 the lengths of light path of the waveguides 9, 10 enclosed by the Mach-Zehnder interferometer type optical circuits 11, 12, and T temperature.
For solving the second problem, it is necessary to redesign the overall structure of the interleaver because its Mach-Zehnder interferometer type optical circuits 11, 12 are too complex in their structure. Similar problems to the above are also observed in certain types of quartz optical waveguide circuits. The refractive index of a quartz material has a positive temperature coefficient whose absolute value is smaller than that of a corresponding polymeric material. Therefore, a known method for preparing an optical waveguide circuit from quartz consists of covering a quartz waveguide core with a polymeric coat whose refractive index has a negative temperature coefficient sufficiently large to cancel the positivity of the temperature coefficient of the quartz waveguide core. However, generally a polymeric material has a refractive index whose temperature coefficient has too large a negative value to cancel the positivity of the temperature coefficient of a quartz material. Naturally, this method can not be applied for the polymeric optical waveguide circuit here concerned.
A known method for compensating for the thermal characteristics of a polymeric waveguide core is to employ a substrate made from a polymer having a high thermal expansion. To put it more specifically, this method consists of employing a polymeric substrate which has a thermal expansion sufficiently high to cancel the negative temperature coefficient of the refractive index of a polymeric waveguide core. However, a substrate made from quartz or silicon generally has a low thermal expansion, and thus as far as based on this method, it will not be possible to integrate optical waveguide circuits on a silicon substrate as in the conventional electronic technology where semiconductor devices are integrated on a silicon substrate.
Accordingly, the object of the present invention is to provide a Mach-Zehnder interferometer-based polymeric waveguide circuit unaffected with the change of ambient temperature which is obtainable by arranging waveguides on a silicon or quartz substrate using conventional IC technology, while maintaining the advantage of low production cost which is the most important impetus for the introduction of polymeric optical waveguide circuits.
An object of the present invention is to provide an optical waveguide coupler circuit device comprising a substrate; a polymeric lower cladding layer formed on the substrate; at least two polymeric optical waveguides formed on the polymeric lower cladding layer; a polymeric upper cladding layer covering the optical waveguides; and plural directional couplers which are obtained by choosing any pair from the at least two optical waveguides, and bringing them close to each other at plural sites, wherein the two paired optical waveguides are configured such that the difference between their effective optical paths spanning between arbitrarily chosen adjacent directional couplers is defined as xcex94L and xcex94L=0.6 to 0.8 xcexcm and each of the plural directional couplers comprises a parallel section at which the two optical waveguides are disposed in parallel with each other.
Another object of this invention is to provide the optical waveguide coupler circuit device wherein the polymeric optical waveguide is made from a polymer having a refractive index of 1.5182 to 1.5667.
Another object of this invention is to provide the optical waveguide coupler circuit device wherein the polymeric lower cladding layer is made from a polymer having a refractive index of 1.5136 to 1.5620.
A further object of this invention is to provide the optical waveguide coupler circuit device wherein the polymeric upper cladding layer is made from a polymer having a refractive index of 1.5136 to 1.5620.
A further object of this invention is to provide the optical waveguide coupler circuit device wherein the length of the two optical waveguides of one directional coupler is chosen to be 0.031 to 0.072 mm while the length of the two optical waveguides of the other directional coupler is chosen to be 0.982 to 1.741 mm.
A further object of this invention is to provide the optical waveguide coupler circuit device wherein the gap between two parallel running waveguides is chosen to be 4.1 to 6.4 xcexcm for both directional couplers.
A further object of this invention is to provide the optical waveguide coupler circuit device wherein each of the optical waveguides is configured to have an oblong cross-section having a width w and a thickness t.
A further object of this invention is to provide the optical waveguide coupler circuit device wherein each of the optical waveguides is configured to have a square cross-section.
A further object hereof is to provide the optical waveguide coupler circuit device wherein each of the optical waveguides is configured to have a square cross-section with a side of 6 to 8 xcexcm.
A further object hereof is to provide the optical waveguide coupler circuit device wherein the substrate is made of a quartz plate.
A further object hereof is to provide the optical waveguide coupler circuit device wherein the substrate is made of a silicon plate.
A further object hereof is to provide the optical waveguide coupler circuit device wherein the substrate is made of a polyimide resin plate. Other objects and advantages will become apparent as this disclosure proceeds.